Here is our solution to override the _get_payslip_lines method in a custom module.

from openerp.osv import osv

from openerp.addons.hr_payroll.report.report_contribution_register import contribution_register_report

import logging

_logger = logging.getLogger(__name__)

class contribution_register_report_pt(contribution_register_report):

    def __init__(self, cr, uid, name, context):

        _logger.error("init NEW BEGIN")

        super(contribution_register_report_pt, self).__init__(cr, uid, name, context)

        self.localcontext.update({

            'report': 'contribution_register_report_pt',

        })

        _logger.error("init new END")

    def _get_payslip_lines(self, obj):

        _logger.error("_get_payslip_lines NEW BEGIN")

        payslip_line = self.pool.get('hr.payslip.line')

        payslip_lines = []

        res = []

        self.regi_total = 0.0

        self.cr.execute("SELECT pl.id from hr_payslip_line as pl "\

                        "LEFT JOIN hr_payslip AS hp on (pl.slip_id = hp.id) "\

                        "WHERE (hp.date_from >= %s) AND (hp.date_to <= %s) "\

                        "AND pl.register_id = %s "\

                        "AND hp.state = 'done' "\

                        "ORDER BY pl.slip_id, pl.sequence",

                        (self.date_from, self.date_to, obj.id))

        payslip_lines = [x[0] for x in self.cr.fetchall()]

        for line in payslip_line.browse(self.cr, self.uid, payslip_lines):

            res.append({

                'payslip_name': line.slip_id.name,

                'name': line.name,

                'code': line.code,

                'quantity': line.quantity,

                'amount': line.amount,

                'total': line.total,

                'number': line.slip_id.number,

            })

            self.regi_total += line.total

        _logger.error("_get_payslip_lines NEW END")

        return res

class wrapped_report_contribution_register_pt(osv.AbstractModel):

    _name = 'report.hr_payroll.report_contributionregister'

    _inherit = 'report.abstract_report'

    _template = 'hr_payroll.report_contributionregister'

    _wrapped_report_class = contribution_register_report_pt